After much debate over the murky boundary of interstellar space, a solar eruption gives scientists the evidence to say Voyager 1 has finally crossed it

By Clara Moskowitz

Voyager 1 was starting to get a reputation as the spacecraft that cried wolf, after scientists repeatedly claimed it was leaving the solar system, only to change their minds and say it wasn't quite there yet. Now researchers say new evidence shows Voyager really has departed the sun's sphere of influence and become the first man-made object to reach interstellar space.

Voyager 1, launched in 1977, is speeding away from us, traveling about 3.5 times the Earth-sun distance every year. The probe is now about 18.2 billion kilometers from Earth, farther away than anything human beings have sent into space. (Its sister spacecraft, Voyager 2, launched more than a month earlier, took a more circuitous path from Earth and is now about 14.8 billion kilometers away.)

Scientists have long been expecting Voyager 1 to exit the bubble of space containing particles from the sun, called the heliosphere, and enter a region where particles are much more plentiful and come from ancient explosions of other stars. But because Voyager 1 has lost its ability to measure this particle plasma, there was no easy way to tell when the transition had occurred.

A boon came from an eruption on the sun in March 2012, which sent waves of solar material out into space. When this ejection reached Voyager 1 13 months later in April 2013, it set the local plasma vibrating. Voyager 1's plasma wave instrument (separate from the defunct plasma particle detector) was able to measure the pitch of these vibrations, which in turn reflected the density of plasma around the spacecraft. The results show that Voyager 1 is surrounded by plasma more than 40 times denser than it encountered earlier, when it was in the heliosphere. "Because there's no other possible conclusion, I think we're forced to and obliged to conclude that we're truly in the interstellar medium," physicist Gary Zank of the University of Alabama in Huntsville said during a press conference today.

Based on abrupt changes in the apparent plasma density around the spacecraft, the researchers were even able to pinpoint August 25, 2012 as the most likely date that Voyager 1 left the solar system, crossing the heliopause, the boundary between the heliosphere and the interstellar medium.

The "little spacecraft that could," in the words of project manager Suzanne Dodd of the NASA Jet Propulsion Laboratory in Pasadena, Calif., is getting old in years and is technologically weak: the average smartphone has thousands of times more memory than the spacecraft. Yet most of Voyager 1's instruments still work, and they continue to send signals back to Earth. The team estimates the probe still has enough power from its plutonium power plant to operate all its instruments through 2020, when it will begin shutting them off one by one, until it goes dark in 2025. That still gives Voyager 1 more than a decade to study the realm of the universe it has entered. "It marks the beginning of a new era of exploration for Voyager: the exploration of the space between the stars," said Ed Stone, Voyager's longtime project scientist.

The new findings represent a turnaround for the Voyager team, which as recently as June predicted the spacecraft may have years to go before it reached interstellar space. But the latest data remove most of the team members' doubts, Stone said.

The new report, published online today in Science, also agrees with the conclusions of a separate paper claiming Voyager 1 had left the solar system, based on magnetic field data, which was published August 14 in The Astrophysical Journal Letters. The lead author of that paper, University of Maryland, College Park, physicist Marc Swisdak, says the two projects are complementary. "I thought the authors make an excellent case for their measurements and their interpretation," Swisdak says.

Over the course of 36 years, between the two of them, the Voyager probes have visited all the outer planets of the solar system and discovered 23 moons around these worlds. They also each carry time capsules in the form of phonograph records loaded with recordings of music, natural sounds, voice greetings in 56 languages, and photographs and diagrams of life on Earth. Voyager 1 is on a course that should eventually take it within 1.6 light-years of a star called AC+79 3888, which lies in the constellation of Camelopardalis. It's due to arrive in about 40,000 years, long after the spacecraft loses its ability to tell whomever is on Earth what comes of that encounter.------------------------------------------------SIDEBAR: A depiction of Voyager 1 crossing into interstellar space. Image: NASA/JPL-Caltech*************************************************